G. Sh. Shekhtman

Crystal structure of the low-temperature form of K3PO4

The crystal structure of the low-temperature form of K3PO4 has been determined for the first time using neutron diffraction (Rietveld method) and Raman spectroscopy: orthorhombic cell (sp. gr. Pnma, Z = 4), lattice parameters a = 1.12377(2) nm, b = 0.81046(1) nm, c = 0.59227(1) nm. The structure is made up of isolated [PO4] tetrahedra, with the potassium ions in between.

Conduction mechanism in the low-temperature phase of KAlO2

The crystal structure of the low-temperature phase of KAlO2 has been studied by neutron diffraction, and its structural parameters have been refined with FullProf software. The TOPOS software package has been used to construct an ionic conduction map and analyze possible potassium ion migration paths. A geometric analysis in conjunction with a qualitative analysis of migration energies lends support to the assumption that low-temperature KAlO2 has one-dimensional potassium ion conduction.

Potassium Aluminate Crystalline Structure and Electroconduction

The crystalline structure of low- and high-temperature modifications of KAlO2 is studied by a neutron diffraction method combined with a full-profile Rietveld analysis. At low temperatures KAlO2 has an orthorhombic structure (spatial group Pbca), which turns tetragonal (spatial group P41212) at 540°C. During a phase transition along the c axis there open new conduction channels with a large cross-section of voids, which is a probable reason for the conduction jump. The presumed anisotropy of conduction in the low-temperature modification disappears during the phase transition.

Crystal structure of the low-temperature forms of cesium and rubidium orthophosphates

The crystal structure of the low-temperature forms of the Cs3PO4 and Rb3PO4 orthophosphates has been determined for the first time by neutron diffraction using the Rietveld method. Cs3PO4 and Rb3PO4 are shown to be isostructural with K3PO4: orthorhombic cell (sp. gr. Pnma, Z = 4); lattice parameters a = 1.23177(6) nm, b = 0.88948(4) nm, c = 0.64197(3) nm for Cs3PO4; a = 1.17362(2) nm, b = 0.81046(1) nm, c = 0.615167(9) nm for Rb3PO4.

Specific features of the crystal structure of polymorphous modifications of KFeO2 and their correlation with ionic conductivity

The structure of potassium ferrite KFeO2 has been studied by high-temperature powder neutron diffraction over a wide temperature range. Based on the structural data, the sizes of potassium cation migration channel have been found in the low- and high-temperature KFeO2 modifications using the TOPOS program package. In the low-temperature form, the channels are nonequivalent, and, as a result, strong anisotropy of the potassium-cation conductivity is observed. During the phase transition, the migration channel sizes increase and are equalized, which is the main cause of the jump in the conductivity.

Rubidium-conducting solid electrolytes in Rb2 − 2xFe2 − xVxO4 system

New solid rubidium-conducting electrolytes based on rubidium monoferrite in the system of Rb2 − 2xFe2−xVxO4 are synthesized and studied. It is found that introduction of V5+ ions causes a drastic decrease in the electronic conductivity component prevalent in pure RbFeO2 with a simultaneous increase in the ionic conductivity. The latter becomes predominant at an increase in the concentration of vanadium. The optimum compositions of the studied electrolytes feature a very high cationic rubidium conductivity (∼1.8 × 10−2 S cm−1 at 200°C, more than 10−1 S cm−1 at 700°C). The results are compared with the data obtained earlier for similar systems based on RbGaO2 and RbAlO2.

Region of existence and electrical properties of La1−x Lix FeO3 solid solution

La1−x Lix FeO3 solid solution prepared by solid-phase synthesis in an air environment at atmospheric pressure is found to exist only for x≲0.1. All single-phase samples are p-type semiconductors. An increase in lithium concentration brings about a decrease in their electrical resistivity and thermopower. The results are discussed in terms of the small polaron (SP) model. The SP concentration and mobility are calculated from data on the electrical resistivity and thermopower in the presence of an antiferromagnetic-paramagnetic phase transition. It is shown that the decrease in electrical resistivity of the samples is connected with the increase in both the concentration and mobility of SPs.

Potassium cationic conduction in K3-xP1-xExO4 (E = S, Cr, Mo, W)

Solid solutions of cubic structure based on K3PO4are synthesized in the systems K3 – xP1 – xExO4(E = S, Cr, Mo, W). Approximate boundaries of single-phase regions and the temperature and concentration dependences of electroconductivity are studied. The transport numbers, measured by a modified Tubandt method, confirm a potassium cationic character of conduction. Electric parameters of the electrolytes are compared with those of sodium and rubidium conducting solid solutions in similar systems. Factors that affect transport properties of synthesized phases are discussed.

Electroconductivity of Solid Electrolytes in the Systems K3–2xM x PO4 (M = Ca, Sr, Ba)

Solid electrolytes K3 – 2xMxPO4 (M = Ca, Sr, Ba) are synthesized and the temperature and concentration dependences of their electroconductivity are studied. Adding calcium and strontium stabilizes the high-temperature β-form of K3PO4 at room temperature, while barium-containing solid electrolytes undergo an eutectoid decomposition below 430°C. Maximum electroconductivity is exhibited by K3 – 2xSrxPO4 (7.1 × 10–3 and 1.25 × 10–1 S cm–1 at 300 and 700°C).

Ion-electron transfer in solid solutions Rb2 − 2xFe2 − xPxO4

The rubidium monoferrite RbFeO2-based solid solutions with the composition Rb2 − 2xFe2 − xPxO4 have been synthesized, and their crystal structure and the temperature and concentration dependences of the total and electron conductivities have been studied. The introduction of P5+ ions has been found to sharply decrease the electron conductivity that prevails in pure rubidium monoferrite and, at the same time, to increase the ionic conductivity. The latter becomes dominant as the phosphorus concentration increases. The maximum rubidium-cation conductivity of the materials under study is ∼3 × 10−2 S/cm at 300°C and ∼3 × 10−1 S/cm at 700°C. The results have been compared with the previously obtained data for similar solid solutions based on rubidium monogallate and monoaluminate.